Recently, services of transmitting and providing data of moving pictures or audio through networks are becoming popular due to development of information communication means represented by Internet. As a data transmission system used in this kind of services, a so-called streaming transmission system is widely used.
FIG. 1 is a block diagram showing a configuration of a moving-picture delivery system performing moving-picture delivery using a network.
In the moving-picture delivery system, moving pictures imaged by a camera 11 are transmitted to a receiving device 14 through a network 13 by a transmission device 12 and displayed on a display 15. Specifically, data of imaged moving pictures is encoded by an encoder 21 and further packetized into RTP (Real time Transport Protocol) packets and the like in the transmission device 12, and then, data is transmitted through the network 13. In the receiving device 14, the RTP packets are received through the network 13, and the data stored in the RTP packets is decoded by a decoder 34 to be displayed on the display 15.
In the transmission device 12, the encoder 21 encodes data by operating based on a frequency of a clock A inputted with the data of moving pictures imaged by the camera 11. On the other hand, in the receiving device 14, the decoder 34 decodes the data received through the network 13 by operating based on a frequency of a clock B generated by a CPU 31 to a VCXO 33. The receiving device 14 is provided with a buffer (not shown) for temporarily accumulating received data. When moving picture delivery is performed for long hours, the buffer overflows in the case where the clock A on the transmission side is faster than the clock B on the receiving side. In contrast to this, when the clock A on the transmission side is slower than the clock B on the receiving side, the buffer will underflow.
As described above, it is necessary that the clock A on the transmission side is synchronized with the clock B on the receiving side in order to perform moving picture delivery using the network in a stable condition.
In the case where moving-picture delivery is performed by using broadcasting waves, clocks are synchronized by comparing a difference value of a PCR (Program Clock Reference) stored in a TS packet (Transport Stream Packet) with a difference value of a STC (System Time Clock) by appropriate operation of a subtractor 61 to a STC counter 66 in a receiving device 51 as shown in FIG. 2. When the system is applied to the moving-picture delivery using the network, it is difficult to completely synchronize clocks on the transmission side and the receiving side because jitter of the TS packet is higher in the network as compared with broadcasting waves.
A commonly-owned invention (for example, Japanese Patent No. 4438878 (Patent Document 1)) proposes a system intended to deliver moving-picture using the network, in which a time stamp of a received TTS packet (Time stamped Transport Stream Packet) is compared with the STC of a receiving device to thereby perform synchronization processing.
As effects of jitter in the network is considered in the above proposal, clocks can be synchronized even in an environment with jitter, however, if information of the STC and the like is not acquired for some reason, it may be difficult to synchronize clocks because the time stamp of the TTS packet is compared with the STC.
Additionally, as a method of synchronizing clocks not using information of the time stamp of the TTS packet, the STC and the like, a method of monitoring accumulated quantity of a buffer which temporarily accumulates receive data to perform clock control in accordance with monitoring results is proposed (for example, refer to JP-A-2003-304224 (Patent Document 2).